There are known methods of detecting fluctuations of the volume of the blood, that is, pulse waves, from an image obtained by imaging a subject. Generally, improvement in detection accuracy is attempted by photographing an image using a light source such as infrared light or photographing an image with an imaging device in close contact with the living body of the subject. However, such a case has demerits such as providing hardware such as a light source and bringing a measurement tool into contact with a living body.
For this reason, it is desired to detect pulse waves without contact between the measurement tool and the living body under environmental light such as sunlight and indoor light. However, the measurement of pulse waves without infrared light or the like incurs large influence of noise, which can possibly decrease the accuracy of detection of pulse waves.
For example, the following signal processor has been presented as an example of a technique for reducing noise. The signal processor is provided with a light-emitting diode that emits red wavelength light and a light-emitting diode that emits infrared wavelength light. With this structure, the signal processor determines a coefficient that minimizes correlation between respective signals obtained by transmission rays of the two light-emitting diodes, and removes a noise component from one signal of the signals using the other signal multiplied by the coefficient. In this processing, the signal processor comprehensively calculates correlation for each of n assumed values, to use the assumed value with the least correlation as a coefficient.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2003-135434
Patent Literature 2: Japanese Laid-open Patent Publication No. 2005-185834
Patent Literature 3: Japanese Laid-open Patent Publication No. 2005-218507
However, the above conventional art increases the processing load because calculation is performed n times to derive a coefficient for reducing noise. In addition, when the number n of the assumed values is reduced to prevent an increase in processing load, the coefficient is diverged from a proper value, which reduces the accuracy of detection of pulse waves.